Calculating Electrical Service Load
Before sizing a panel or service entry, total the wattage of all circuits. The National Electrical Code (NEC) requires specific demand factors for different load types.
Load Calculation Formula
Total Load (W) = Σ (device watts × hours/day)
Panel Amps = Total Watts / Voltage
Single-phase 240V: A = W / 240
Three-phase 208V: A = W / (√3 × 208) = W / 360Typical Residential Loads
Lighting (whole home): 2000-4000 W
HVAC (3-ton A/C): 3500 W
Electric water heater: 4500 W
Electric range: 8000-12000 W
Clothes dryer: 5000-6000 W
EV charger (Level 2): 7200 W
Example home total: ~28,000 W
Service: 28,000 / 240 = 117A → 150A serviceNEC Demand Factors
- General lighting: 3 VA/ft² (33 VA/m²)
- Small appliance circuits: 1500 VA each (minimum 2)
- Dryer: 5000 VA minimum
- Apply 100% demand to first 10kW, 40% to remainder
Calculate electrical loads: Free Electrical Load Calculator
Electrical Load Formulas
- Single-phase power: P = V × I × PF (VA = V × I)
- Three-phase power: P = √3 × V_LL × I × PF
- Current from power: I = P / (V × PF) for single-phase
- Load diversity factor: Reduces connected load to account for not all loads running simultaneously
Sizing Electrical Systems
Electrical system design starts with load calculation: listing all loads (kW or kVA), applying demand factors and diversity factors, and summing to find peak demand. This determines transformer ratings, main switchboard sizing, cable cross-sections, and protection device ratings. For residential properties, a 100 A service (230 V single-phase) provides 23 kVA — sufficient for most homes. Commercial buildings are typically three-phase with ratings from 400 A (280 kVA) to multi-MVA switchboards. Generator sizing requires starting current of motors (typically 6–8× running current) in addition to running loads.
Frequently Asked Questions
What is the difference between connected load and demand load?
Connected load is the sum of nameplate ratings of all installed equipment — if every single load ran simultaneously at full power. Demand load (maximum demand) is the highest load actually drawn in a period (typically 15 or 30 minutes). Demand factor = demand load / connected load. For a typical office, demand factor is 0.6–0.8 — not all lights and equipment run simultaneously at full power.
How do I read a 3-phase electricity bill?
Three-phase supplies deliver higher power (√3 × greater than single-phase at same voltage and current) and are used by commercial and industrial customers. The bill typically shows: kWh consumed (energy), kVA maximum demand (peak apparent power, charged as a capacity fee), power factor (with penalties if low), and reactive energy (kVARh). The capacity charge incentivises managing peak demand, not just total consumption.
What is a load schedule and when is it required?
A load schedule is a systematic tabulation of all electrical loads in a building, grouped by distribution board, showing: load description, kW, quantity, demand factor, design current, cable size, and protection rating. Building regulations and BS 7671 require load schedules for all but the simplest installations. They are essential for commissioning, future modifications, and fault diagnosis.